As a material of optical elements to be used in optical systems of various cameras such as cameras, film integrated type cameras and video cameras, an optical glass or an optical transparent resin is used. Optical glasses are excellent in heat resistance, transparency, size stability, chemical resistance, etc., and there are various materials with different refractive indexes (nD) or Abbe numbers (νD). However, optical glasses have problems of high material costs, bad molding processability and low productivity. In particular, significantly advanced techniques and high costs are required for processing for obtaining an aspherical lens to be used for aberration correction, and this is a major obstacle from a practical viewpoint.
Meanwhile, advantageously, optical lenses made of optical transparent resins, particularly thermoplastic transparent resins can be mass-produced by injection molding, and in addition, an aspherical lens can be easily produced therefrom. Such optical lenses are currently used for camera lenses. Examples thereof include a polycarbonate made of bisphenol A, polystyrene, poly-4-methylpentene, polymethyl methacrylate and an amorphous polyolefin.
However, when using an optical transparent resin as an optical lens, in addition to the refractive index and Abbe number, transparency, heat resistance and low birefringence are also required, and therefore it has a weak point that portions for use thereof are limited depending on the balance of characteristics of the resin. For example, polystyrene has low heat resistance and high birefringence, poly-4-methylpentene has low heat resistance, polymethyl methacrylate has a low glass transition temperature, low heat resistance and a low refractive index, and therefore use areas of these materials are limited, and a polycarbonate consisting of bisphenol A has weak points such as high birefringence, and therefore portions for use thereof are limited.
Regarding the optical design of optical units, it is known that there is a method in which chromatic aberration is corrected by combined use of a plurality of lenses with different Abbe numbers. For example, chromatic aberration is corrected by combined use of a lens made of a cycloolefin resin having a relatively high Abbe number and a lens made of a polycarbonate resin consisting of bisphenol A having a low Abbe number. However, there is a difference between the hygroscopic expansion coefficient of the cycloolefin resin and the hygroscopic expansion coefficient of the polycarbonate resin, and when forming a lens unit by combining these two types of lenses, the difference between the sizes of the lenses is made at the time of water absorption depending on an operating environment of a smartphone or the like. Due to the difference of the expansion rate, performance of the lens is impaired.
Patent Documents 1 to 3 describe polycarbonate copolymers containing a perhydroxy dimethanonaphthalene skeleton, but since the dihydroxymethyl group is positioned at 2,3-position in each case, the strength is low, and therefore these materials are not suitable for use in optical lenses.
Moreover, recently, it has been remarkably desired to realize wider angles of, for example, front cameras and rear cameras of smartphones. However, there is a problem that partial chromatic aberration is increased in association with wider angles. For this reason, materials having excellent optical characteristics with heat resistance and a higher Abbe number are desired.